Talking to Girls About Sex

I HAVE A 4YR OLD LITTLE GIRL AND I AM TRYING TO TALK TO HAR ABOUT SEX ALREADY IT STARTED OFF WITH WHEN MY HUSBAND AND U WERE CLOSING OUR BEDROOM DOOR WHEN WE HEARD BOTH OF OUR KIDS SAY (WHY ARE YOU GUYS GONA HAVE SEX) IT WAS A JAW DROPING MOMENT WE BOTH DIDN'T KNOW HOW TO REACT. THEN IT STARTED TO GET WORSE OUR KIDS WERE IN THE CLOSET WITH THERE UNDERWEAR BOY AND GIRL 5 AND 4 YERAR OLD. THEN MY HUSBAND AND I SEEN OUR DAUGHTER IN THE CLODET WITH HER BABY DOLLS NAKED. I WANA KNOW HOW TO GO ABOUT THIS. I AM A STRONG BELIEVER IN GOD AND I FEEL THERE IS A SPIRT OF LUST IN HER OR SHE IS REAL OBZERVENT.